The primary purpose of this study is to assess the benefits and risks of neonatal screening for cystic fibrosis (CF) with a randomized controlled investigation of all children born in Wisconsin. In the first phase of the project, we evaluated immunoreactive trypsinogen assay (IRT) comprehensively to determine if it is helpful for CF patients. More recently, we have applied molecular genetics technology to develop a two-tiered testing system combining the IRT measurement with DNA mutation analysis. The primary outcome variables assess nutritional status and severity of bronchopulmonary disease.